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P0300

glenn-ho

By glenn-ho
in Ask A GM Technician

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glenn-ho Enthusiast

glenn-ho

Posted
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This is long but please stay with me!

 

Threw a blinking CEL on the way back from vacation. Stopped shut it down and popped the hood but everything looked ok. Started it back up with a rough idle, loss of power but not bad, and a constant CEL. Drove it home and when up to speed it drove OK. Got home pulled out the scanner and pulled a P0305 misfire cyl 5.

 

Changed the cap, rotor, plugs and wires with no change. Pulled the plugs and did a compression check and found cyl #5 at 140lbs and the rest 185lbs.

 

Pulled left head found cyl # 5 exhaust valve bent. Decide to pull the other. Sent them out. Replaced the valve springs, valve seals and three exhaust valves. Pulled the upped intake off and cleaned the lower intake along with the spider. I have changed the oil every 3,000 miles or sooner. I had no sludge in the heads or cam valley. I could not believe how clean it was for 186,000 miles. My friend the machinist also could not believe it. He also stated that it had no wear to the valve guides. Valve seat on cyl # 5 was good also.

 

OK let get to the thick of it. Put it back together checking the distributor for gear wear and slop which it had none before assembly. Fired it up and now is it showing a p0300 random misfire. Checked fuel pressure and it a constant 60lbs. Changed the fuel pump at about 140,000 with a ACdelco due to fuel level gauge not reading.

 

Checked compression and have 185lbs all cyl's. Checking the plugs cyl #1 and #4 are clean and all others are black?

 

Don't no where to go from here? Can a crank sensor or cam sensor be bad and only set a P0300? Or do you think the spider is bad?

glenn-ho Enthusiast

glenn-ho

Posted
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pulled the upper intake off last night and did not find any fuel from leaking. Verified by Hooking up wiring and fuel lines to spider with a pressure gauge on the fuel line and turned the key on. No leaking from meter body, regulator, or injectors. Does not tell me the poppets are good but no leaking. Also note have good vacuum checked before taking off upper intake.

glenn-ho Enthusiast

glenn-ho

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How can I prove it Out? I put over 300,000 on a 1997 with a 4.3l and never had any problems like this. Also why would it have been OK before I removed the heads and now all of a sudden I need a new spider. I always add a bottle of techron every oil change.

ChevyTech77 Senior Enthusiast

ChevyTech77

Posted
Posted

Tried.

 

How else can you have a fuel rich situation with good compression and good spark WITHOUT it being something in the injection side? I've seen literally hundreds of these trucks with the same issue. Just because the poppets don't leak (which in fact they rarely do) doesn't mean they aren't spraying like crap, it just means that you aren't going to have a cold fuel flooding situation during a start that's all. If you don't believe me, that's fine I mean hey I am trying to diagnose your vehicle over the internet so...... :lol:

glenn-ho Enthusiast

glenn-ho

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I am leaning that way to. What I can't understand is the plugs I pulled out before the bent valve had nice clean burn. Never had any problems at all. Only after I replaced the heads and started it back up did I run into this problem. I guess stranger things can happen right? I appreciate your help Chevytech77. Thank you.

glenn-ho Enthusiast

glenn-ho

Posted
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Posted

FINAL OUTCOME - Installed the updated MFI injector kit and it cured everything. Thank you Chevytech77!!! no more P0300 and the truck runs even better then it did before.

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